1. Field of the Invention
The present invention relates to an absolute position measurement apparatus, and more particularly to an absolute position measurement apparatus that outputs signals whose phase changes in accordance with changes in position or angle, such as an encoder or a laser interferometer.
2. Description of the Related Art
Encoders, in general, measure an amount of light transmitting through a scale provided with light transmitting parts and light shielding parts at a certain pitch.
Laser interferometers, in general, split a laser beam into two light beams, one of which is reflected by a mirror provided in a movable part while the other is reflected by a mirror provided to a fixed part, so as to measure intensity of these interference lights.
In either of an encoder or a laser interferometer, the phase of a sine wave output signal changes in accordance with changes in position or angle of an object to be measured. Among these, a measurement apparatus that outputs two-phase signals whose phases are different from each other by 90°, which are approximated by a sine function and a cosine function, is known. A measurement apparatus that outputs three-phase signals whose phases are different at every 120° is also known. That is, encoders and laser interferometers generally output plural signals having different phases.
Signal processing devices used in encoders and laser interferometers perform processing such as arctangent calculation to the plural output signals having different phases to determine the phases of the output signals. Such arithmetic processing enables fine position information of the object to be measured to be obtained, and counting the wave number of the output signals provides rough position signals, which are connected to serve as position signals.
With a measurement apparatus such as an encoder or a laser interferometer, the rough position or angle of the object to be measured is measured by determining a wave number of the signal. However, which point is the origin, i.e., an absolute position or angle, cannot be determined based only on output signals of common encoders or laser interferometers.
Therefore, encoders, in general, are additionally provided with a means of outputting an origin signal. However, with an encoder having a small scale pitch in particular, it is difficult to form an origin signal that is output only in a small range.
As for laser interferometers, a technique is employed for setting a rough origin using other means such as a limit switch to obtain an origin signal. However, outputting an origin signal with high reproducibility in a small range corresponding to one cycle of an interference signal is a technical challenge, and even if it were overcome, the apparatus would be expensive.
Japanese Patent Laid-Open No. 2007-33318 discloses a laser interferometer capable of measuring an origin. The laser interferometer disclosed in this publication is configured using two light sources having different wavelengths and coherence. Japanese Patent Laid-Open No. 2007-33318 also discloses a laser interferometer in which an interference signal having low coherence has sufficient intensity, and a point where the phase difference between both interference signals is zero is determined as the origin.
However, an apparatus for measuring an origin with a method disclosed in Japanese Patent Laid-Open No. 2007-33318 would have an extremely complex structure. An interference signal obtained from a light source with low coherence generally has a large distortion. Because of this, the point where the phase difference between both interference signals is zero is not fixed to one point. Accordingly, it would be difficult to actually form an absolute position measurement apparatus based on this method.